Post acceleration grid devices



June 3, 1958 c, DU 2,837,689

POST ACCELERATION GRID DEVICES I Filed Dec. 1, 1955 POST ACCELERATIONGRIDDEVICES (Iharles Dufour, Paris, France, assignor to CompagnieGenerale de Telegraphic Sans Fil, a corporation of France ApplicationDecember 1, 1955, Serial No. 550,413

Claims priority, application France December 2, 1954 2 Claims. (Cl.315-914) The present invention relates to post acceleration grid devicesfor oscilloscopes and other tubes, such as the Graphecon. A device ofthis type has been describedin the co-pending application Serial No.497,311 filed on March 28, 1955, now abandoned, by Charles Dufour for:Improvements in Oscilloscopes Systems. It comprises a thin grid of hightransparency, positioned at a small distance from the tube target (forinstance to 30 mm.) and subjected to a low voltage with respect to theanode voltage of 10,000 volts. It can be proved that such a device willnot introduce any geometrical distorsion into the system, whileoperating with a much higher sensitivity than that of conventionalpost-accelerating cathoderay tubes. The device has, however, thefollowing disadvantages: the impact of the electrons on thepostaccelerating grid extracts secondary electrons, a certain number ofwhich, in oscilloscopes for instance, are picked up by the screen whichis the seat of a much higher potential than that of the grid. Anelectron in the beam will then produce a small luminous patch on thescreen, instead of a luminous point, causing the curves or imagesrepresented to be slightly blurred. It has been thought possible toavoid the blurring effect by increasing the transparency of the grid orby reducing the number of meshes. The results of these experiments,however, were unsatisfactory.

The object of the invention is a post-accelerating grid device usable inoscilloscopes or other tubes which does not present this disadvantage.

The device according to the invention comprises a post-accelerating gridand a second grid placed between the post-accelerating grid and thetarget, said second grid being spaced from said post-accelerating gridby a distance which is small with respect to its distance from thetarget, said second grid being impressed with a voltage slightly lowerthan that of the first grid.

According to one feature of the invention, the wires of one of the gridsare inclined at an angle of 45 with respect to the wires of the othergrid.

According to another feature of the invention, the

two grids form a single unit comprising a grid-shaped insulating supporthaving a metal film coated on either side thus forming first and secondgrids.

The invention will be better understood with the aid of the appendeddescription and with reference to the annexed drawings, wherein:

Fig. 1 shows a diagram of a cathode ray tube embodyin g thepost-accelerating device according to the invention.

Fig. 2 is an explanatory diagram of operation of the tube of Fig. 1.

Fig. 3 shows the preferred relative position of themeshes of the twogrids.

Fig. 4 shows a cross-sectional view of a detail of a preferredembodiment of the post-accelerating device according to the invention.

In Fig. l, the cathode ray tube represented comprises a glass envelope1, coated with a metal film 2 raised to a voltage of l500volts withrespect to the'electron emisgrid 3. This screen grid'5 is raised to avoltage of 1,000

volts, i. e. at 500 volts with respect to grid 3; Grids 3 and 5 are verythin and comprise 20 square meshes per mm. Besides these-elements, thetube according to the invention comprises all other parts common toconventional'cathode-ray tubes, i; e.: theabove-mentioned electrongunl16 with focusing and' accelerating electrodes (not shown),and-horizontal and'vertical deflecting plates 6, 7 and 8, 9. A directcurrent source 10 provides the necessary voltages required by thevarious elements.

The cathode-ray tube of Fig. 1 operates as follows i (for a betterunderstanding of the operation, Figure 2 has been drawn on a largerscale) Screen 4, post-accelerating grid 3 and screen grid 5 are, asmentioned, raised to a voltage of 11,500, 1,500 and 1,000 volts,respectively. The trajectory of a primary electron emitted by gun "16appears at points 11, 13 and 15 of grids 3 and 5 and screen 4respectively; the trajectories of the secondary electrons generated bysaid electron through the impact on the two grids are shown at 11-12 andlit-14. The screen grid 5 will thus repel the secondary electronsleaving the, post-accelerating grid 3 with zero initial velocity. Theseelectrons will fall back on the latter grid 3. Also, saidpost-accelerating grid 3, close to screen grid 5, will intercept thesecondary electrons produced through the impact of the primary beam onthe latter. reach the screen and the post-acceleration process will notbe substantially modified thereby.

A moir or watered silk effect on the oscilloscope could eventuallyoccur, were one of the grids to become slightly displaced with respectto the other, if the wires constiv tuting the meshes of said two gridswere parallel to one another. This is avoided by arranging the two gridsin the manner shown in Figure 3, wherein one of the grids is representedin solid lines and the other in dotted lines, the wires of one of thegrids being inclined at an angle of 45 with respect to the wires of theother grid.

Finally, grids 3 and 5 may also form a single unit as shown incross-section in Figure 4: a grid-shaped support 17, made of insulatingmaterial, is mounted in the tube, perpendicular to the axis of gun 16,between the gun 16 and the screen 4. On the side of the support facingthe gun, a film of conductive material is deposited, constituting grid3. On the opposite side, a second film is deposited constituting grid 5.

Perfect parallelism between the grids 3 and 5 is thus insured. Anyshort-circuit resulting fromth'e application of different potentials tosaid two grids and any shift of one of the grids with respect to theother are Patented June 3, 1958 No secondary electron will thus 3 tivewith respect to said cathode and highly negative with respect to saidtarget, and means for causing an electron beam issued from said cathodeto scan said tar- .get, said tube further comprising an additional griddisposed between said post-accelerating grid and said target,substantially nearer to the post-accelerating grid than to the target,and means for raising said additional grid to a slightly negativepotential with respect to said postaccelerating grid, whereby secondaryemission electrons from both grids are prevented from reaching saidtarget, said grids being constituted of fine wires, the wires of saidfirst grid being disposed at an angle of 45 with respect to the wires ofsaid second grid.

2. A monochromatic cathode ray tube of the type including a cathode, atarget, a post-accelerating grid disposed in the vicinity of saidtarget, means for raising said post-accelerating grid to a potentialslightly positive with respect to said cathode and highly negative withrespect to said target, and means for causing an electron beam issuedfrom said cathode to scan said target, said tube further comprising anadditional grid disposed between said post-accelerating grid and saidtarget, substantially nearer to the post-accelerating grid than to thetarget, and means for raising said additional grid to a slightlynegative potential with respect to said post-accelerating grid, wherebysecondary emission electrons from both grids are prevented from reachingsaid target, further comprising a grid-shaped insulating support havinga first and a second sides, said first and second sides beingrespectively coated with a first and second films of metal constitutingrespectively said first and said second grids.

References Cited in the file of this patent UNITED STATES PATENTS2,315,367 Epstein Mar. 30, 1943 2,580,250 Smith Dec. 25, 1951 2,602,145Law July 1, 1952 2,692,532 Lawrence Oct. 26, 1954 2,728,024 Ramberg Dec.20, 1955 2,734,146 Noskowicz Feb. 7, 1956

